C# Recursive Increment Decrement not working as parameters

Question

I am using recursion to add two numbers together, By adding 1 to the first input one at a time until I have reached the value of the second. Why does this work...

        private static int AddMethod(int input1, int input2)
    {
        if (input2 == 0)
        {
            Console.WriteLine(input1);
            return (input1);
        }
        else
        {
            input1++;
            input2--;
            return AddMethod(input1, input2);
        }

    }

But not this..

    private static int AddMethod(int input1, int input2)
    {
        if (input2 == 0)
        {
            Console.WriteLine(input1);
            return (input1);
        }
        else
        {
            return AddMethod(input1++, input2--);
        }

    }

I am using Visual Studio 2010 and .Net 4.0

What doesnt work about it? it looks more like you want `return AddMethod(++input1, --input2);` — Sayse, Jul 29 '14 at 12:35
There's a good explanation of what is going on at [What is the difference between i++ and ++i?](http://stackoverflow.com/questions/3346450/what-is-the-difference-between-i-and-i/3346729#3346729). — Andrew Morton, Jul 29 '14 at 12:39
Useful [related question](http://stackoverflow.com/questions/484462/difference-between-i-and-i-in-a-loop) — Sayse, Jul 29 '14 at 12:39

score 6 · Accepted Answer · answered Jul 29 '14 at 12:36

6

Because return AddMethod(input1++, input2--); first passes your inputs, and THEN increments and decrements.

Try return AddMethod(++input1, --input2);

answered Jul 29 '14 at 12:36

Benjamin Diele

1,177
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score 2 · Answer 2 · answered Jul 29 '14 at 12:36

Post fix increment works by first "assigning" the value, then incrementing the value.

Compare:

int a = 1;
int b = 1;

int x = a++;
int y = ++b;

So in your case, the value you pass to AddMethod is the unchanged value, it modifies the value of input1 and input2 after they are passed.

Patrick Hofman · Answer 3 · 2014-07-29T12:45:59.583

2

Because the ++ and -- operators are executed after passing the values as parameters to the function.

Your code:

return AddMethod(input1++, input2--);

Is equal to:

int result AddMethod(input1, input2);
input1++;
input2--;
return result;

Instead of all this, you could use:

return AddMethod(++input1, --input2);

edited Jul 29 '14 at 12:45

answered Jul 29 '14 at 12:36

Patrick Hofman

153,850
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C# Recursive Increment Decrement not working as parameters

3 Answers3